Hybrid hard disk drive (HDD) as one of storage devices include one or more rotating magnetic disks combined with nonvolatile solid-state (e.g., flash) memory. Generally, a hybrid HDD has both the capacity of a conventional HDD and the ability to access data as quickly as a solid-state drive, and for this reason hybrid HDDs are expected to be commonly used in laptop computers.
However, a drawback of the NAND-based memory cells in solid-state drives and hybrid HDDs is that such memory cells are only operable for a limited number of program/erase cycles. Furthermore, at higher temperatures, data retention of NAND memory cells is greatly reduced. For example, when single-level cell (SLC) NAND memory cells are elevated to a temperature greater than 70 to 80° C., even short-term retention of data of these NAND cells may become unreliable, and they cannot be considered a dependable nonvolatile data storage medium. This is because data stored in NAND cells may be lost at anytime under such conditions. For a hybrid HDD that includes multi-level cell (MLC) NAND memory, reliability is even more of an issue, since MLC cell reliability may be compromised after fewer program/erase cycles and at lower temperatures than SLC cells.
Reads from and writes to NAND memory are much faster than reads from and writes to a magnetic disk, so a significant advantage of hybrid HDDs over conventional HDDs is that the NAND memory included in a hybrid HDD can be used as a large read and write cache that reduces the frequency of relatively time-consuming disk accesses. However, implementation of a write cache in NAND memory necessitates storing the only copy of data associated with recently received write commands in NAND memory cells. As noted, under some conditions NAND memory cells may be unsuitable for long-term storage of the only copy of data in a data storage device. Thus, the improved performance provided to a hybrid HDD by a NAND-based write cache is offset by the fact that, under certain conditions, NAND memory cells are inherently less reliable than other storage media in the hybrid HDD.